U.S. patent application number 13/067651 was filed with the patent office on 2011-12-22 for speed or torque probe for gas turbine engines.
This patent application is currently assigned to WESTON AEROSPACE LIMITED. Invention is credited to Robert Michael George Bodin.
Application Number | 20110308331 13/067651 |
Document ID | / |
Family ID | 42582822 |
Filed Date | 2011-12-22 |
United States Patent
Application |
20110308331 |
Kind Code |
A1 |
Bodin; Robert Michael
George |
December 22, 2011 |
Speed or torque probe for gas turbine engines
Abstract
There is provided a variable reluctance sensor for sensing the
speed or torque of a shaft in a gear box or gas turbine engine,
comprising a magnetic pole piece a conductive wire wrapped around
the pole piece a housing surrounding the pole piece, the housing
having a front face and at least one side wall, wherein, in use,
the front face is positioned proximate to an object to be sensed
wherein the pole piece is rigidly fixed to the side wall of the
housing. This arrangement reduces microphony in the sensor.
Inventors: |
Bodin; Robert Michael George;
(Hampshire, GB) |
Assignee: |
WESTON AEROSPACE LIMITED
Farnborough
GB
|
Family ID: |
42582822 |
Appl. No.: |
13/067651 |
Filed: |
June 16, 2011 |
Current U.S.
Class: |
73/862.193 |
Current CPC
Class: |
G01L 3/109 20130101;
G01P 1/026 20130101; G01P 3/488 20130101 |
Class at
Publication: |
73/862.193 |
International
Class: |
G01L 3/02 20060101
G01L003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 22, 2010 |
GB |
1010497.4 |
Claims
1. A variable reluctance sensor for sensing the speed or torque of
a shaft in a gear box or gas turbine engine, comprising: a magnetic
pole piece; a conductive wire wrapped around the pole piece; a
housing surrounding the pole piece, the housing having a front face
and at least one side wall, wherein, in use, the front face is
positioned proximate to an object to be sensed, wherein the pole
piece is rigidly fixed to the side wall of the housing.
2. A variable reluctance sensor according to claim 1, wherein the
pole piece is rigidly fixed to a plurality of points on the
housing.
3. A variable reluctance sensor according to claim 1, wherein the
pole piece is welded to the housing.
4. A variable reluctance sensor according to claim 1, wherein the
pole piece is mechanically fixed to the housing.
5. A variable reluctance sensor according to claim 1, further
comprising a rigid intermediate structure wherein the pole piece is
rigidly fixed to the rigid intermediate structure, and the rigid
intermediate structure is rigidly fixed to the housing.
6. A variable reluctance sensor according to claim 1, wherein the
pole piece extends through and is fixed to the front of the
housing.
7. A variable reluctance sensor according to claim 6, wherein the
pole piece is welded to the front face of the housing.
8. A variable reluctance sensor according to claim 1, wherein the
pole piece extends through the front face of the housing but is not
fixed to the front face of the housing.
9. A variable reluctance sensor according to claim 1, wherein the
front face of the housing is substantially less rigid than the side
wall of the housing
10. A variable reluctance sensor according to claim 1, further
comprising a permanent magnet, the permanent magnet positioned
within the housing adjacent to the pole piece.
11. A variable reluctance sensor according to claim 10, wherein the
permanent magnet is rigidly fixed to a side wall of the
housing.
12. A variable reluctance sensor according to claim 10, wherein the
pole piece has a front end adjacent to the front face of the
housing and a rear end positioned adjacent to the permanent magnet,
and wherein the rear end of the pole piece is fixed to the side
wall of the housing.
13. A variable reluctance sensor according to claim 1, wherein the
pole piece is formed in one piece and comprises a longitudinally
extending shaft around which the conductive wire is wrapped, and a
rear end which is rigidly fixed to the housing.
14. A variable reluctance sensor according to claim 1, wherein the
conductive wire is held in place relative to the pole piece by a
packing material surrounding the conductive wire and a portion of
the pole piece.
15. A gas turbine engine comprising: a rotating shaft; a phonic
wheel mounted to the shaft for rotating with the shaft; and a
variable reluctance sensor according to claim 1, wherein the front
face of the housing is positioned proximate to the phonic
wheel.
16. A variable reluctance sensor for sensing the speed or torque of
a shaft in a gear box or gas turbine engine, comprising: a magnetic
pole piece; a conductive wire wrapped around the pole piece; a
permanent magnet adjacent to the pole piece; a housing surrounding
the pole piece and the permanent magnet, the housing having a front
face and at least one side wall, wherein, in use, the front face is
positioned proximate to an object to be sensed, wherein the pole
piece is rigidly fixed to the side wall of the housing.
17. A variable reluctance sensor for sensing the speed or torque of
a shaft in a gear box or gas turbine engine, comprising: a magnetic
pole piece; a conductive wire wrapped around the pole piece; a
housing surrounding the pole piece, the housing having a front face
and at least one side wall, wherein, in use, the front face is
positioned proximate to an object to be sensed, wherein the pole
piece extends through the front face of the housing and the front
face of the housing is substantially less rigid than the side wall
of the housing.
18. A variable reluctance sensor according to claim 17 wherein the
pole piece is fixed to the front face of the housing.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to variable reluctance probes
used for measuring the speed and torque applied to rotating shafts.
Probes of this type are typically found in gas turbine engines or
in gear boxes used in aircraft.
BACKGROUND TO THE INVENTION
[0002] Variable reluctance sensors are used to monitor both the
speed of rotating shafts and the torque loading on shafts in gas
turbine engines and gear boxes connected to gas turbine engines.
For example, FIG. 1 illustrates an assembly for monitoring the
torque loading on a power transmission shaft between a gas turbine
and the power gearbox that drives the propeller.
[0003] When a load is applied to a power transmission shaft it will
twist. For a known modulus of elasticity and at a constant
temperature, the amount of twist (A) is proportional to the torque
transmitted. This basic principle is used to measure torque.
[0004] The assembly shown in FIG. 1a comprises two intermeshed
phonic wheels 10, 11, attached to the rotating shaft 12 but at
points longitudinally spaced from each other. The torque
transmitted by the shaft is calculated by measuring the time
difference between the passage of the teeth of the two phonic
wheels past a variable reluctance sensor. FIG. 1 b illustrates a
sensor 14 positioned adjacent to the teeth of a phonic wheel
17.
[0005] From FIG. 1a, it can be seen that the phase wheel 10 is
attached directly to the shaft 12. The reference wheel 11 is
attached to a reference tube 13 mounted concentric with the torque
shaft 12, and is fixed to the torque shaft at one end leaving the
reference wheel 11 free. When the shaft 12 is loaded it will twist
but the unloaded reference tube 13 will not, so that the phase
wheel 10 moves relative to the reference wheel 11. As a result, the
reference wheel becomes a datum from which to calculate the angle
of twist, .theta.. As the phonic wheels 10, 11 are intermeshed
movement of the phonic wheels with respect to each other will be
evident by the time intervals of the passage of phonic teeth on the
phonic wheels past the sensor. The teeth on the phase phonic wheel
will move closer to the teeth on the reference phonic wheel in the
direction of rotation and twist. At the same time the distance
between the trailing teeth of the reference phonic wheel in respect
to the phase phonic wheel teeth will increase. This is illustrated
in FIGS. 2a and 2b, which are schematic representations of the
relative positions of the teeth on each wheel in an unloaded state
and a loaded state respectively. The distance of `tm` is always
smaller than `ts` so that the control system can differentiate
between `tm` and `ts` when the phonic wheels start to rotate.
[0006] The distance between the phonic wheel teeth will be seen as
the distance between the zero crossovers in the A/C signal produced
by the variable reluctance sensor. The change in distance in the
zero cross over will be directly proportional to the angle of the
twist of the shaft (.theta.) and so the torque transmitted by the
shaft. A typical clean signal waveform from a variable reluctance
sensor sensing the passage of the teeth can be seen in FIG. 4, with
time on the x-axis and voltage on the y-axis.
[0007] The same basic principle is equally applicable for the
measurement of rotational speed via a phonic wheel. The time
between the passings of adjacent teeth past a sensor can be
measured to provide a signal from which rotational speed can be
calculated.
[0008] Both the conventional type of variable reluctance sensor,
where many turns of a conductive wire are wrapped around a magnetic
pole piece, and the transformer type as described in U.S. Pat. No.
7,148,679, where a few turns of a primary turn of conductive wire
are wrapped around magnetic pole piece, can be used. FIG. 3 is a
schematic cross section of a typical construction of a variable
reluctance sensor.
[0009] The sensor of FIG. 3 comprises a magnetic pole piece 30
around which an electrically conductive wire 31 is wound. A
permanent magnet 32 is positioned adjacent a back face 30a of the
pole piece 30. The front face of the pole piece 30b is, in use,
located proximate to the phonic wheel or wheels being sensed, as
shown in FIG. 1b. The pole piece 30, conductive wire 31 and
permanent magnet are all held in a housing 33. An encapsulation
material 34, typically a powder or an epoxy resin, is used to fill
the space between the housing 33 and the pole piece 30, magnet 32
and conductive wire 31. The housing 33 is fixed to another part of
the turbine engine (not shown) and ensures that the front face of
the pole piece is correctly positioned relative to the phonic wheel
or wheels. The housing also provides protection from the harsh
environment found inside gas turbine engines.
[0010] As each tooth of the phonic wheels passes close to the front
face of the pole piece there is a change in the magnetic flux
experienced by the conductive wire 31, due to the change in the
reluctance of the magnetic circuit consisting of the pole piece 30,
the phonic wheel and the air gap between the two. The changing
magnetic flux results in a variable current induced in the
conductive wire 31, from which the timing of the passage of the
teeth on the phonic wheels past the pole piece can be
determined.
[0011] In both torque and speed measurement, it is important that
the waveform produced by the variable reluctance sensor is very
clean and there is no noise or additional modulations, known as
microphony, on the signal waveform. FIG. 4 illustrates a clean
waveform. In contrast, FIG. 5 shows a waveform that is not
acceptable as there is significant noise 50 present. If the noise
amplitude exceeds the trigger threshold of the engine controls, the
torque or speed measuring system will not function properly as the
noise will be interpreted as an additional zero crossing, and in
extreme circumstances the controls may shut the engine down if the
torque or speed measurement is a primary engine function.
[0012] One major cause of noise in the output from variable
reluctance sensors, producing the additional modulations or
microphony, is vibration from the surrounding environment.
Vibration can be created from many areas of a gas turbine engine
and surrounding ancillary equipment, such as the power gear box
where large intermeshing teeth create vibration, out of balance
shafts, bearings and compressor/turbine blades and discs.
[0013] The reason that vibrations cause noise in the output signal
is the affect that they have on the pole piece. Vibration in the
sensor environment can cause stress in the pole piece that alters
its magnetic permeability. The change of the magnetic permeability
of a material when subjected to a mechanical stress is known as the
Villari effect. The stress energy created in the pole piece causes
strain, which affects the permeability and so alters the reluctance
of the device. As the pole piece has conductive wires wrapped
around it and a magnet or coil attached at one end, the change in
reluctance will cause a change in the magnetic flux around the pole
piece, inducing an additional electrical current in the conductive
wire wrapped around the pole piece. This additional induced current
is the source of noise or microphony in the output signal. This
effect is more noticeable at high vibration frequency levels
because of the greater rate of change of permeability of the pole
piece.
[0014] A problem with existing sensors, as illustrated in FIG. 3,
is that any forces exerted on the pole piece 30 by the magnet 32
and/or the surrounding encapsulation medium or the housing front
face result in strain energy in the pole piece 30. This strain
energy changes the permeability of the pole piece, creating EMF in
the conductive wire 31, which produces additional, unwanted
modulations in the waveform, as shown in FIG. 5.
[0015] In a sensor as illustrated in FIG. 3, the inventors have
found that there are two main mechanisms by which strain is
generated in the pole piece. First, vibration from the surrounding
environment causes the magnet to vibrate. The permanent magnet is
relatively massive and vibrations of the magnet produce stress in
the pole piece as the magnet pushed against it, resulting in
microphony in the coil. Second, vibration from the surrounding
environment causes vibration of the housing front face, which is
transferred to the pole piece as strain energy, resulting in
microphony in the coil.
[0016] The encapsulation material does, to some extent, reduce the
transfer of vibration to the pole piece, and epoxy resin as an
encapsulation material has proven to be the most effective
material. However, at high frequency and high temperature there is
still significant noise in the sensor output as a result of
environmental vibrations. One factor is that, at the high
temperatures found in gas turbine engines, the epoxy resin used as
an encapsulating material is relatively soft.
[0017] It is an object of the present invention to substantially
reduce the sensitivity of variable reluctance sensors, suitable for
use in gas turbine engines, to noise resulting from environmental
vibrations.
SUMMARY OF THE INVENTION
[0018] The present invention is defined in the appended claims to
which reference should be made. Preferred features of the invention
are set out in the dependent claims.
[0019] In a first aspect, the invention comprises a variable
reluctance sensor for sensing the speed or torque of a shaft in a
gear box or gas turbine engine, comprising:
[0020] a magnetic pole piece;
[0021] a conductive wire wrapped around the pole piece;
[0022] a housing surrounding the pole piece, the housing having a
front face and at least one side wall, wherein, in use, the front
face is positioned proximate to an object to be sensed,
[0023] wherein the pole piece is rigidly fixed to the side wall of
the housing.
[0024] By fixing the pole piece to the side wall of the sensor
housing, strain in the pole piece as a result of external vibration
is significantly reduced. The strain is effectively transferred to
the side wall of the housing rather than along the length of the
pole piece. Preferably, the pole piece comprises a longitudinal
shaft proximate to the front face of the housing, wherein the
conductive wire is wrapped around the longitudinal shaft, and a
head end remote from the front face of the housing, wherein the
head end is fixed to the side wall of the housing.
[0025] Preferably, the pole piece is rigidly fixed to a plurality
of points on the housing. By connecting the pole piece to the
housing at a plurality of points or over an extended area, the
transfer of strain can be increased. "Rigidly fixed" in this
context means more than simply held in place through an
interference fit with other components of the sensor or with an
encapsulation material. It requires a positive fixing means. The
pole piece may be fixed to the side wall or side walls of the
housing by any suitable means, such as welding, brazing or by using
some mechanical fixing, such as a screw fitting. It is also
possible to include a rigid intermediate structure between the pole
piece and the housing to provide the rigid fixing. The pole piece
is then directly fixed to the intermediate structure and the
intermediate structure directly fixed to the housing. This may have
advantages in the assembly of the sensor.
[0026] The pole piece may extend through the front face of the
housing and it may be advantageous that the pole piece is not fixed
to the front face of the housing, so as to minimise the transfer of
strain from the front face of the housing. Whether a connection
between the pole piece and the front face of the housing is
required depends on whether a seal is required isolating the
interior of the housing and the environment in which the sensor is
operating.
[0027] Alternatively, or in addition, the front face of the housing
may be made substantially less stiff that the side wall of the
housing. By having a front face that is able to flex, less stress
is exerted on the pole piece under external vibration.
[0028] It is important that the conductive wire does not move
relative to the pole piece. A packing material, such as fibre glass
tape covered in varnish, or an epoxy resin, may be provided around
the conductive wire and the portion of the pole piece around which
the wire is wrapped, to secure the wire relative to the pole
piece.
[0029] The variable reluctance sensor may further comprise a
permanent magnet within the housing and adjacent to the pole piece.
In this case, typically, the pole piece has a front end adjacent to
the front face of the housing and a rear end positioned adjacent to
the permanent magnet, with the rear end fixed to the side wall of
the housing. The permanent magnet may be rigidly fixed to a side
wall to minimise relative movement between the magnet and the pole
piece.
[0030] In a second aspect, the invention comprises a variable
reluctance sensor for sensing the speed or torque of a shaft in a
gear box or gas turbine engine, comprising:
[0031] a magnetic pole piece;
[0032] a conductive wire wrapped around the pole piece;
[0033] a housing surrounding the pole piece, the housing having a
front face and at least one side wall, wherein, in use, the front
face is positioned proximate to an object to be sensed,
[0034] wherein the pole piece extends through the front face of the
housing and the front face of the housing is substantially less
rigid than the side wall& the housing.
[0035] Optional or preferred features described in relation to the
first aspect may equally be applied to the second aspect of the
invention.
[0036] In a third aspect, the invention comprises a gas turbine
engine comprising:
[0037] a rotating shaft;
[0038] a phonic wheel mounted to the shaft for rotating with the
shaft; and
[0039] a variable reluctance sensor according to the first or
second aspect, wherein the front face of the housing is positioned
proximate to the phonic wheel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] Embodiments of the invention will now be described in
detail, by way of example, with reference to the accompanying
drawings, in which:
[0041] FIG. 1a is perspective, partially cut away, view of a phonic
wheel assembly for torque measurement;
[0042] FIG. 1b illustrates a variable reluctance sensor positioned
adjacent to a phonic wheel;
[0043] FIG. 2a is a schematic representation of the teeth of the
phonic wheels of FIG. 1a with no load applied to the power
transmission shaft;
[0044] FIG. 2b is a schematic representation of the teeth of the
phonic wheels of FIG. 1a with a load applied to the power
transmission shaft;
[0045] FIG. 3 is a schematic cross section of a prior variable
reluctance sensor;
[0046] FIG. 4 is a clean output waveform from a variable reluctance
sensor;
[0047] FIG. 5 is a noisy output waveform from a variable reluctance
sensor;
[0048] FIG. 6 is a schematic cross section of a prior variable
reluctance sensor with an indication of the forces on the pole
piece as a result of environmental vibrations;
[0049] FIG. 7 is a schematic cross section of a first variable
reluctance sensor in accordance with the invention;
[0050] FIG. 8 is a schematic cross section of a second variable
reluctance sensor in accordance with the invention;
[0051] FIG. 9 is a schematic cross section of a third variable
reluctance sensor in accordance with the invention; and
[0052] FIG. 10 is a schematic cross section of a fourth variable
reluctance sensor in accordance with the invention.
DETAILED DESCRIPTION
[0053] The basic arrangement for a variable reluctance sensor for
detecting speed or torque in a gas turbine engine or gear box
connected to a gas turbine engine has been described previously
with reference to FIGS. 1 to 3. The problem of unwanted microphony
in the output signal from such variable reluctance sensors,
resulting from strain in the magnetic pole piece 30, has also been
described.
[0054] FIG. 6 illustrates a variable reluctance sensor as shown in
FIG. 3, but illustrating the direction of the strain in the pole
piece 30, which affects the nature of the output signal from
conductive wire 31. The pole piece is formed from a soft magnetic
material, is roughly T-shaped in cross-section and has a
longitudinally extending shaft around which the conductive wire is
wound and a rear head, against which the permanent magnet 32 is
positioned. Current induced in the wire 31 as result of changes in
the reluctance of the circuit formed by the magnetic pole piece,
the phonic wheel, and air gap between the phonic wheel of a pole
piece are used to determine torque and/or speed. Longitudinal
strain in the longitudinal shaft of the pole piece indicated by
arrow 60 alters the magnetic permeability of the pole piece, in the
region around which the conductive wire is wound. Any change in
permeability also causes a variable reluctance in the magnetic
circuit, and so induces an additional current in the conductive
wire 31.
[0055] As described above, the longitudinal strain in the pole
piece results from vibration of the sensor assembly, and in
particular is transferred to the pole piece both from the permanent
magnet 32 and from the front face of the housing 33b. Typically,
although the pole piece extends through the front face of the
housing, it is attached to the front face of the housing by a
welded or brazed joint. The pole piece is therefore effectively
trapped between the front face of the housing 33b and permanent
magnet 32.
[0056] FIG. 7 illustrates a variable reluctance sensor in
accordance with the present invention. The sensor of FIG. 7 differs
from the prior sensor shown in FIG. 6, in that the flat head of the
pole piece 70a extends to and is rigidly fixed to the side walls
73a of the housing 73. The front face of the housing 73b is also
made substantially thinner, and therefore less massive and
substantially more flexible (or less rigid) than the side walls of
the housing 73a.
[0057] As a result of these modifications, strain in the pole
piece, resulting in the force exerted by the permanent magnet 72 is
transferred to the side walls 73a of the housing, rather than all
being concentrated along the longitudinal shaft 70b of the pole
piece, as illustrated by arrows 75. This results in a significant
reduction in the strain within the pole piece and hence a reduction
in the noise in the output of the sensor. The fact that the front
face of the housing 73b is more flexible, means that less force is
applied to the pole piece from the front face of the housing. This
also reduces noise in the output from the sensor.
[0058] The pole piece may be fixed to the front face 73b of the
housing, for example by welding, if it is required to have a good
seal so as to protect the interior of the housing from the outside
environment. However, if a good seal is not required, the pole
piece may advantageously not be connected to the front face 73b of
the housing, so that the housing exerts no significant force on the
pole piece when it vibrates.
[0059] The materials used to make the sensor shown in FIG. 7 are
substantially the same as those used conventionally in variable
reluctance sensors of this type. The pole piece is formed in a
single piece and is made of soft magnetic material, such as soft
iron or ferritic stainless steel.
[0060] The encapsulation material is a high temperature epoxy
resin, a powdered material, or a ceramic paste or silicone
rubber.
[0061] The permanent magnet may formed from any suitable material
such as Samarium Cobalt, Alcomax.TM., Hycomax.TM., and Alnico.TM..
The conductive wires are typically formed from insulated copper or
copper alloy winding wire. The housing is formed from stainless
steel.
[0062] In the embodiment shown in FIG. 7, the pole piece 70 is
welded to the side walls of the housing 73. Advantageously, the
pole piece is welded to the housing at a plurality of locations, or
over an extended area. All that is required is that there is a
sufficient space for the conductive wires to pass through the pole
piece or between the pole piece and the housing to connect to
external processing electronics. Typically, the housing is
cylindrical, with a round cross-section, and the pole piece is
welded to the side walls of the housing around its circumference.
Ideally, the connection of the pole piece to the housing is
symmetrical about the longitudinal axis of the pole piece.
[0063] Alternatively, other means of rigidly fixing the pole piece
to the housing may be used, such as a screw fixing or a clamp
fitting. The pole piece might also or alternatively be glued to the
side walls of the housing.
[0064] FIG. 8 shows an alternative embodiment of a variable
reluctance sensor in accordance with the invention. In the sensor
of FIG. 8, the pole piece is magnetised by a primary circuit 80
wound around the pole piece, through which an alternating current
is passed. The permanent magnet of the embodiment shown in FIG. 7
can therefore be removed and is simply replaced by further
encapsulation material 74. This type of sensor is sometimes known
as a transformer type sensor. Even in the absence of a massive
permanent magnet within the housing, the features of fixing the
pole piece to the side walls of the housing and of making the front
face of the housing more flexible and less massive, still have
significant benefit in reducing strain energy in the pole piece.
Again, the pole piece may be anchored only to the side walls and
not the front face of the housing, or alternatively may be anchored
to both the side walls and the front face of the housing, as
required.
[0065] FIGS. 9 and 10 illustrate two further embodiments of a
variable reluctance sensor in accordance with the invention,
similar to the embodiment of FIG. 7, but that provide for simple
manufacture and assembly. In the sensor of FIG. 9 the pole piece 90
has a ring 91 attached to it. The ring is formed from a stainless
steel or other material that remains rigid at high temperature. The
ring 91 may be brazed, glued or welded to the pole piece base. This
ring 91 is then be attached to the housing 93 by welding,
mechanical attachment or adhesive as before.
[0066] FIG. 10 shows a sensor similar to that shown in FIG. 9, but
with a cup 101 in place of a ring. The cup protrudes further back
than the ring of FIG. 9 and forms a cup around the pole piece 100
and magnet 102. The cup 101 provides support and helps to locate
the magnet 102 onto the pole piece base so that the base of the
pole piece and magnet act as one unitary mass. The cup 101 is
attached to the housing 103 by welding, mechanical attachment or
adhesive, in a similar manner as before.
[0067] In both the embodiment of FIG. 9 and FIG. 10 there still
needs to be sufficient space for conductive wires to pass the pole
piece base and magnet for electrical connections.
[0068] Variable reluctance sensors in accordance with the present
invention are able to measure speed or torque parameters reliably
in harsh, vibrating environments, both at low and high vibration
frequencies, and at high temperatures. They are also protected from
instantaneous shock loads which would otherwise induce strain in
the pole piece.
* * * * *